<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .wrap{
            display: flex;
        }
        .box1,.box3{
            width: 100px;
            height: 100px;
            background-color: #bfa;
        }
        .box2{
            flex: 1;
            background-color: #ff6700;
            height: 100px;
        }
    </style>
</head>
<body>
    <div class="wrap">
        <div class="box1"></div>
        <div class="box2"></div>
        <div class="box3"></div>
    </div>
</body>
</html>